Disc drives are typically used as primary data storage devices in modern computer systems and networks, due to the efficient and cost-effective manner in which large amounts of computerized data can be stored and retrieved. Disc drives of the present generation have data storage capacities measured in excess of several gigabytes (GB) and can be used alone (as in a typical personal computer configuration) or in multi-drive data storage arrays (as with an internet network server or a mainframe computer).
A typical disc drive comprises a plurality of rigid magnetic storage discs which are axially aligned and arranged about a spindle motor for rotation at a constant high speed (such as around 10,000 revolutions per minute). An array of read/write heads are provided to transfer data between tracks of the discs and a host computer in which the disc drive is mounted. The heads are mounted to a rotary actuator assembly and are controllably positioned adjacent the tracks by a closed loop servo system.
The servo system primarily operates in one of two selectable modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head away from the initial track and toward the destination track. A velocity control approach is used whereby the velocity of the head is repeatedly measured and compared to a velocity profile defining a desired velocity trajectory for the seek. Once the head has settled on the destination track, the servo system enters a track following mode of operation wherein the head is caused to follow the destination track until the next seek operation is performed.
Both track seeking and track following operations typically require generation of a position error signal (PES) which gives an indication of the radial position of the head with respect to the tracks on the disc. In high performance disc drives, the PES is derived from either a prerecorded servo disc with a corresponding servo head (a dedicated servo system), or from servo information that is embedded on each recording surface among user data blocks at predetermined intervals (an embedded servo system).
The head provides the servo information to the servo system which generates the PES with a magnitude that is typically equal to zero when the head is positioned over the center of the track ("on track"), and is nominally linearly proportional to a relative misposition distance ("off track") between the head and the center of the track, with a polarity indicative of radial off track direction. To provide stable operation, the transfer function relating PES to actual radial misposition should be constant and independent of distance off track in the presence of variations in head signal amplitude, linear recording bit density, head to media spacing and head skew angle.
Recently, magneto-resistive (MR) heads have supplanted prior use of thin film heads due to the superior magnetic recording properties associated with MR heads. Generally, an MR head includes a magneto-resistive read element characterized as having a baseline direct current (DC) electrical resistance that changes when subjected to magnetic fields of selected orientation. An MR head can detect previously recorded data in response to variations in voltage measured across the MR head when a read bias current of predetermined magnitude is passed through the MR element. MR heads have allowed disc drives of the present generation to achieve track densities greater than about 4,000 tracks per centimeter (about 10,000 tracks per inch).
Although MR heads facilitate ever greater levels of magnetic recording densities, there are nevertheless disadvantages associated with such heads when used as position transducers, due to nonlinear readback response with respect to position. This nonlinearity is due to a variety of factors, including MR element bias, sensitivity differences across pole face geometry of the MR elements, and aggravation of the inherent nonlinearity of end fringing fields, as the width of a typical MR element readback gap is typically appreciably less than the width of the tracks on the associated disc.
Historically, PES nonlinearity primarily affected track seeking performance, producing track arrival over-shoot or under-shoot which prolonged arrival settling time. PES nonlinearity did not so adversely affect track following performance (i.e., did not induce significant track misregistration, or TMR), because the track center for reading and writing, defined by servo bursts of the servo information (typically referred to as A, B, C and D bursts) nominally coincided with a null response of positioning burst signals A-B=0.
With MR heads, however, PES nonlinearity also affects TMR control during track following because the MR element is physically separated from the write element of the head by a distance governed by the design of the head; thus, inaccuracies can arise as a result of head skew (with respect to the disc) and head fabrication tolerances. Because the write gap and the read gap are at slightly different radial positions, the PES must be operated at a position A.noteq.B to properly center the write gap over the track. Further, nonuniformity in readback magnetic field sensitivity generally produces a positional shift in null response of bursts A-B=0. Unless properly characterized and compensated, the nonlinearity will produce a discrepancy from desired positioning and can introduce instabilities into the servo system.
PES nonlinearity due to the use of MR heads can not only therefore adversely affect seek performance, but track following performance as well. Such effects can further serve as a limit on achievable track densities (and hence data storage capacities), due to the TMR budget necessary to minimize interference with (such as overwriting of) adjacent tracks.
Accordingly, there remains a continual need in the art for improvements whereby disc drive performance can be optimized through minimizing the effects of position error signal nonlinearities.